Integrated circuits (ICs) are typically mounted in packages to protect the sensitive integrated circuitries from environmental influences. However, one disadvantageous side effect that may be observed is that mounting the integrated circuitry in a package exerts mechanical stress on the semiconductor material. Mechanical stress on integrated circuits changes electronic parameters, such as the magnetic sensitivity of Hall plates or the resistance of resistors. Mechanical stress changes the mobility and the scatter factor of charge carriers, which causes lifetime drifts of resistances, transistor parameters, and the magnetic sensitivity of Hall plates (known as piezo-resistivity effect, piezo-MOS effect, piezo-junction effect, and piezo-Hall effect).
Lifetime drift of mechanical stress originates from changes of the thermomechanical properties of the package constituents (e.g. aging or chemical reactions in the mold compound or swelling of the mold compound due to moisture ingress), and typically cannot be avoided. Silicon Hall sensors are known to suffer from a long term drift in magnetic sensitivity between 1% and 4% depending upon the degree of moisture in the mold compound of the package.
Vertical Hall effect sensors are also affected by the lifetime drift of mechanical stress. Vertical Hall sensors differ from planar Hall sensors or “Hall plates” that vertical Hall sensors are capable of measuring surface-parallel components of the magnetic field. They allow therefore relatively easy conception of single-chip multi-axial magnetic sensors compared to solutions using horizontal Hall plates. The modern trend in the field of Hall sensors is to integrate them into electronic circuitry for signal processing. The great advantage of these vertical Hall-effect sensors is that they can be manufactured in a standard CMOS process without additional post-processing.
Hence, mechanical stress within the active region of a vertical Hall sensor may lead to a gain error over the lifetime of the Hall sensor caused by mechanical stress in vertical Hall sensors (caused by packaging, humidity changes, soldering, . . . ). As a result, a change of sensitivity and/or a change of switching points may be observable. Typically, it is relatively difficult or not possible at all to adjust for these changes by programming.